Flexible molded polyurethane foam requires mechanical crushing to open foam cells and prevent shrinkage and to improve dimensional stability of the foam pad. Current mechanical methods for cell opening consist mainly of crushing, vacuum rupture or time pressure release.
Upon demold, mechanically crushing and breaking the polyurethane foam cells enables the polyurethane foam to be more dimensionally stable. Another method of breaking the cells is vacuum crushing which involves drawing a vacuum on the finished polyurethane foam causing cell rupture. The overall effect of these methods is reduced foam shrinkage.
Other mechanical attempts have been made to achieve dimensionally stable foam, such as decreasing cycle production times. For example, demolding the polyurethane foam in three minutes as compared to four minutes will dramatically improve the dimensional stability. However, this can lead to deformation, tearing, or distortion of the polyurethane foam due to undercure.
Another method for producing dimensionally stable foam is time pressure release (TPR). TPR comprises opening the mold during the curing process to release the internal pressure and then reclosing for the duration of the cure time. The sudden release of the internally generated pressure bursts the cell windows, thereby obtaining an open cell foam. The effect of TPR can be varied by performing the TPR at different stages in the curing process, and by varying the length of time the mold is opened before reclosing. This pressure release is performed only once during the cure time of each polyurethane foam. This process may cause corner blowouts, surface defects, and dimensional distortions and, if the defect is severe enough, it will result in scrap polyurethane foam. These discrepancies are considered minor compared to the effect of TPR and its ability to open the foam. Additionally, upon demold the foam must also be subjected to mechanical or vacuum crushing since TPR does not completely provide the necessary energy to totally open cells in the foam.
Mechanical methods usually result in incomplete or inconsistent cell opening and require a flexible molded foam producer to invest in additional machinery. A chemical method for cell opening would be preferred.
U.S. Pat. No. 4,929,646 discloses preparing flexible polyurethane foams using certain high molecular weight, high functionality poly(oxyethylene) compounds as cell openers and softeners.
U.S. Pat. No. 4,751,253 discloses a cell opening, dimensionally stabilizing additive for making flexible polyurethane foam which additive comprises an ester reaction product of a long chain acid with polyethylene or polypropylene glycols and/or contains free acid to provide for desired acid value.
U.S. Pat. No. 4,701,474 discloses the use of acid grafted polyether polyols, such as acrylic acid grafted poly(alkylene oxides), as reactivity controllers in the production of polyurethane foam.
U.S. Pat. No. 4,785,027 discloses preparing polyurethane foams in the presence of polyether mono- or diacids, with the acid functional groups at the ends of the polymer chain. Such polyether acids reportedly delay the initial reaction rate without increasing foam tightness.
U.S. Pat. No. 5,489,618 discloses polyurethane foam prepared in the presence of a salt of a tertiary amine and a carboxylic acid having hydroxyl functionality as a catalyst. Reportedly, the flexible foams produced are more dimensionally stable and have a decreased tendency to shrink.
U.S. Pat. No. 5,179,131 discloses that the addition of mono- or dicarboxylic acids to polyurethane foam formulations made using polyisocyanate polyaddition polymer polyol dispersions (PIPA) results in a reduction of foam shrinkage. Functional groups attached to the acid are either alkyl or alkylene.
U.S. Pat. No. 4,211,849 discloses a process for making open celled, crosslinked foams using as the crosslinker a crystalline polyhydroxy material having at least three hydroxy groups.
EP 471 260A discloses the use of organic acids or their alkali salts for the production of open cell polyurethane foam. It is stated that incorporation of these materials gives foam with markedly lower forced to crush values.
WO 9506673 discloses alkali metal and alkaline earth metal salts of alkyl and alkenyl succinic acids as catalysts for production of polyurethane and/or polyurea foam.